1. Field of the Invention
The invention pertains to the inspection of optical media, such as ophthalmic lenses. More particularly, it relates to a method and system for inspecting ophthalmic lenses using light at wavelengths that are substantially absorptive to the ophthalmic lens, e.g. light at select ultraviolet (UV) or infrared (IR), wavelengths when the lens is a soft contact lenses. In an image of the lens generated therefrom, thickness changes caused by cosmetic flaws or thickness changes that are deliberately designed into the ophthalmic lens, manifest as measurable changes in transmitted light intensity. In practice, the invention reduces the number of false rejections by enabling a significant increase in the level of discrimination between cosmetic flaws critical to lens quality and extraneous artifacts which are not; and permits verification that intentional changes in lens thickness meet specification. The invention is especially adaptable to high speed, automated inspection. In certain embodiments involving soft contact lenses, the invention can be implemented without removing the lenses from the molds in which they are made, thus creating greater efficiencies in the inspection process.
2. Description of the Prior Art
Various techniques for inspecting ophthalmic lenses exist. Initial endeavors relied upon human inspectors to visually examine the lens for defects. These usually-entailed placing the lens under magnification or projecting it onto a screen whereafter the inspector would manually search for irregularities by e.g. varying the field of focus to examine the lens for flaws at different depths. The labor intensive and subjective nature of human operator inspections impelled interest in automating aspects of the process. For example, methods have been developed whereby an image of the lens is generated and electronically evaluated for defects. These commonly take advantage of the fact that light, under certain conditions, scatters in a manner that can be qualitatively assessed when it encounters a lens irregularity. These methods ordinarily employ white light because of its convenient availability, and because they do not require light of a specific spectral profile for functionality. Indeed, such methods avail themselves of the fact that the ophthalmic lens is effectively transparent to the inspection light, and operate by manipulating the light beam before and/or after it passes through the lens to extract the necessary optical information that is subsequently analyzed to assess for flaws.
The more common of such lens inspection methods involve imaging the lens under either dark field (DF) illumination or bright field (BF) illumination conditions. In dark field, the manipulation of the light beam entails partially blocking the light source so that only light rays whose path through the ophthalmic lens have been disrupted (e.g. by a lens flaw or irregularity) will be imaged. In a dark field system, anything that causes a change in the optical path of light rays traversing the lens will be greatly enhanced and will appear in the image as a bright spot on a dark field. An example of a dark field illumination lens inspection technique is described in Canadian Patent Application 2057832. In bright field, the ophthalmic lens is typically fully illuminated by the light source, i.e. the light source is not partially blocked. Such a technique is described in U.S. Pat. No. 5,500,732, which illuminates the lens at specified grey levels to which the imaging camera is sensitive; irregularities in the ophthalmic lens then appear in the image as dark spots against a bright field.
While some of these techniques have proven industrially useful, there nevertheless exists an impetus to continually improve contrast in the lens image to further distinguish defective from non-defective areas of the lens, as well as discrimination between cosmetic flaws that are fatal to lens quality and extraneous lens artifacts that are not. Because white light is employed in these prior techniques, irrelevant lens detail is often imaged while important detail is lost. This can result in too many false rejections of lenses. Investigations into other types of lens inspection techniques include that which is reported in: U.S. Pat. No. 5,633,504 wherein the lens is imaged under conditions of fluorescence. Such a method is limited, however, by the types of materials that can be analyzed by fluorescence.
Furthermore, these earlier techniques are inadequate to measure deliberate changes in lens thickness, such as occurs with toric thin zones in certain types of contact lenses, to verify that they meet specification. These toric thin zones are intentionally provided and allow for rotational positioning on the lens on the eyeball.
Moreover, the practices of the prior art require improvements in the lenses employed in the imaging apparatus. Conventionally, the imaging lenses utilized in imaging devices known heretofore have had flat focal surfaces. Representative of art in this regard is U.S. Pat. No. 5574554 which uses a combination of collimated white light and a telecentric imaging lens. The difficulty with such configurations, however, is that the ophthalmic lens being imaged is oftentimes curved (e.g. to fit the eyeball in the case of a contact lens),not flat. In particular, the art typically utilizes a negatively powered lens element having a small beam diameter in front of the flat imaging plane in the imaging device. The result is a compromise in focus, to the diminishment of the inspection process.
Hence the ophthalmic lens inspection art continually seeks to explore and develop techniques that will heighten discernible contrasts between fatal cosmetic flaws and non-fatal extraneous artifacts, which techniques will lead to a decrease in the number of false rejections and be easily implemented leading to inspection efficiencies.
The present invention is directed to the inspection of ophthalmic lenses utilizing an absorptive technique. In one aspect, the invention is directed to a method for inspecting ophthalmic lenses which comprises illuminating an ophthalmic lens with light comprising a wavelength that is substantially absorptive to said ophthalmic lens; detecting an image of at least part of said ophthalmic lens only from light that is at said substantially absorptive wavelength which has been transmitted through said lens; and analyzing said image for changes in the intensity of the light that is at said absorptive wavelength which has been transmitted through said lens. In the practice of the invention, the changes in transmitted light intensity as aforesaid are caused by changes in thickness of the ophthalmic lens. These can be caused by cosmetic flaws, irregularities or can be due to the design of a particular ophthalmic lens. The present invention also pertains to a system for such inspections. In another aspect especially useful where the ophthalmic lens has curvature (e.g. a contact lens), the invention relates to an imaging lens assembly for an imaging sensor, the imaging lens assembly having a curved focal surface to match the curvature of the ophthalmic lens, thus ameliorating focus and consequent image analysis. The invention is further directed to a lens assembly that directs inspection light onto the surface of the ophthalmic lens at angles of incidence that are substantially normal to said surface. The invention is also directed to a lens carrier having a specific frustroconical well design; and to a specific apodizer.